Pilot patterns for ofdm systems with multiple antennas

ABSTRACT

The present invention relates to orthogonal frequency-division multiplexing (OFDM) communication systems with four transmit antennas and one or more receive antennas, and in particular to methods for inserting scattered pilots (SPs) into the transmit signals of such OFDM systems, for estimating channel properties on the basis of the scattered pilots, a multi-antenna OFDM transmitter, and an OFDM receiver. In this context, it is the particular approach of the present invention to keep the same SP pattern like in the single-transmitter case, to partition the pilots into as many subsets as there are transmitters (transmit antennas), and to interleave these subsets both in time and in frequency. In this manner, the granularity of pilots of the same subset is reduced. This offers increased flexibility in designing the scattered pilot patterns and greater accuracy of the estimated channel properties.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to OFDM (orthogonal frequency-divisionmultiplexing) communication systems with a plurality of (e.g., four)transmit antennas and one or more receive antennas, and in particular tomethods for inserting scattered pilots into the transmit signals of suchOFDM systems, for estimating channel properties on the basis of thescattered pilots, a multi-antenna OFDM transmitter, and an OFDMreceiver.

Description of the Related Art

Orthogonal Frequency-Division Multiplexing (OFDM) is a digitalmulti-carrier modulation scheme, which uses a large number ofclosely-spaced orthogonal sub-carriers. Each sub-carrier is modulatedwith a conventional modulation scheme (such as quadrature amplitudemodulation) at a low symbol rate, maintaining data rates similar toconventional single-carrier modulation schemes in the same bandwidth.

The primary advantage of OFDM over single-carrier schemes is its abilityto cope with severe channel conditions, for example, attenuation of highfrequencies at a long copper wire, narrowband interference andfrequency-selective fading due to multipath, without complexequalization filters. Channel equalization is simplified because OFDMmay be viewed as using many slowly-modulated narrowband signals ratherthan one rapidly-modulated wideband signal. Low symbol rate makes theuse of a guard interval between symbols affordable, making it possibleto handle time-spreading and eliminate inter-symbol interference.

In OFDM communications, scattered pilots (SP) are typically used forchannel estimation and equalization. Scattered pilots are complex OFDMcells with known phase and amplitude arranged in frequency and timeaccording to a defined pattern. The pilots are typically selected from abinary alphabet, e.g., {+1; −1} and are boosted in power compared to thedata cells.

FIG. 1 shows an example of such a pattern in the form of a diagonalgrid, as it is used in DVB-T (digital video broadcasting terrestrial),i.e., a digital broadcasting standard based on OFDM (cf. the ETSIStandard ETS 300 744, “Digital Broadcasting Systems for Television,Sound and Data Services; framing structure, channel coding andmodulation for digital terrestrial television”). The pilots areindicated by the black circles, whereas the data cells are indicated byopen circles.

The scattered pattern shown in FIG. 1 is characterized by twoparameters: D_(s) is the distance between SPs that are adjacent alongthe time axis in a pilot-bearing subcarrier, and D_(k) is the distancebetween two SP-bearing subcarriers that are adjacent along the frequencyaxis. These two parameters are also referred to as the SP spacing intime and frequency, respectively. In this type of pattern, SPs arepresent in every OFDM symbol and the distance between two SPs in asymbol is D_(s)D_(k). The present invention builds on such a SP pattern.In DVB-T, D_(s)=4 and D_(k)=3, as shown in FIG. 1.

Since the channel (the state of the channel) usually varies (fades) intime, due to Doppler variations, and in frequency, due to multi-pathdelay, the SP pattern must be dense enough to sample the channelvariations along both axes as required by the sampling theorem. TheD_(s) parameter defines the sampling along the time axis, whereas theD_(k) parameter defines the sampling along the frequency axis.

The channel estimation process consists of two steps. First, the channelis estimated at the SP positions by dividing the received value by theknown pilot value (the reference signal). Second, the channel estimatesfor the other cells are computed by interpolating between the estimatesat the SP positions. The interpolation is conceptually two-dimensional,but can be practically performed by first interpolating in time then infrequency, as shown in FIG. 9. Moreover, interpolation can be combinedwith noise reduction in order to improve the accuracy of the estimate.

In order to increase the communication reliability, multipletransmitters operating in parallel in the same frequency band can beused. This is referred to in the art as multiple-input single-output(MISO), when there is one receiver, or multiple-input multiple-output(MIMO), when there are multiple receivers. As in the single-transmittercase, channel estimation is required for coherent demodulation.

In the general MIMO case, the channel between each transmitter and eachreceiver must be estimated. A MIMO configuration for 4 transmitters and2 receivers is shown in FIG. 10 as an example. We can express thereceived signal vector y as a function of the transmitted signal vectorx and the channel matrix H as shown in the following Math. 1.

$\begin{matrix}{\begin{bmatrix}y_{1} \\\vdots \\y_{M}\end{bmatrix} = {\begin{bmatrix}h_{11} & \ldots & h_{1\; N} \\\vdots & \ddots & \vdots \\h_{M\; 1} & \ldots & h_{MN}\end{bmatrix}\begin{bmatrix}x_{1} \\\vdots \\x_{N}\end{bmatrix}}} & \left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack\end{matrix}$

wherein N is the number of transmitters, e.g., N=4, and M is the numberof receivers. All quantities are complex valued.

A channel estimation process is performed by each receiverindependently. For channel estimation purposes the number of receiversis therefore irrelevant. The signal seen by a receiver can be written asshown in the following Math. 2.

$\begin{matrix}{y = {\sum\limits_{n = 1}^{N}\; {h_{n}x_{n}}}} & \left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack\end{matrix}$

Each receiver produces estimates of the N channel components h₁, . . .h_(N) at each OFDM cell based on the values received at the SPlocations.

Any implementation of an MISO or MIMO system based on OFDM thus has todefine (i) how the scattered pilots are to be encoded so that the Nchannel components can be easily estimated in the receiver and (ii) howthe scattered pilots are to be arranged in time and frequency.

The key idea for estimating the channel components is to employdifferent SPs for different transmitters. In order to be able toestimate the individual channel components, the SPs are partitioned intoas many subsets as there are transmitters. All pilots belonging to asubset are multiplied by a constant coefficient that depends on thesubset and the transmitter (transmit antenna). In the four-transmitter(four-transmit antenna) case, there are 16 coefficients, which can beexpressed as a 4×4 matrix, as shown in FIGS. 12A and 12B. The rowscorrespond to the transmitters (transmit antennas) and the columns tothe SP subsets.

According to FIG. 12A, a pilot that is transmitted by a transmit antennan and belongs to subset m is multiplied by a coefficient C_(mn). Forexample, a result of multiplying a pilot by one of the four coefficientsC₁₁ (for the transmit antenna 1), C₁₂ (for the transmit antenna 2), C₁₃(for the transmit antenna 3), and C₁₄ (for the transmit antenna 4) isused as an SP to be allocated to signals transmitted in subset 1.

Regarding the values of the coefficients, there is one necessary andsufficient condition that must be met in order to be able to separatethe channel components in the receiver: the coefficient matrix must befull rank, i.e. invertible. The justification is readily apparent if wewrite the received values at the pilot locations as in the followingMath. 3.

$\begin{matrix}{y_{m} = {\left( {\sum\limits_{n = 1}^{N}\; {h_{n}c_{mn}}} \right)p_{m}}} & \left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack\end{matrix}$

In the above Math. 3, p_(m) is the original value (beforemultiplication) of a pilot in subset m, y_(m) is the received value atthe location of the said pilot, h_(n) is the channel between transmitter(transmit antenna) n and receiver, and c_(mn) is the constant pilotcoefficient for subset m and transmitter (transmit antenna) n. Forsimplicity the channel noise has not been considered here.

The original pilot values p_(m) are irrelevant. Denoting the y_(m)/p_(m)ratio by e_(m), the equation of the above Math. 3 can be expressed inmatrix form shown in the following Math. 4.

$\begin{matrix}{\begin{bmatrix}e_{1} \\\vdots \\e_{M}\end{bmatrix} = {\begin{bmatrix}c_{11} & \ldots & c_{1\; N} \\\vdots & \ddots & \vdots \\c_{M\; 1} & \ldots & c_{MN}\end{bmatrix}\begin{bmatrix}h_{1} \\\vdots \\h_{N}\end{bmatrix}}} & \left\lbrack {{Math}.\mspace{14mu} 4} \right\rbrack\end{matrix}$

The channel estimates for the channels h₁ to h_(N) can be computed byleft multiplying the e_(m) estimates by the inverse of the coefficientsmatrix as shown in the following Math. 5.

$\begin{matrix}{\begin{bmatrix}h_{1} \\\vdots \\h_{N}\end{bmatrix} = {\begin{bmatrix}c_{11} & \ldots & c_{1\; N} \\\vdots & \ddots & \vdots \\c_{M\; 1} & \ldots & c_{MN}\end{bmatrix}\begin{bmatrix}e_{1} \\\vdots \\e_{M}\end{bmatrix}}} & \left\lbrack {{Math}.\mspace{14mu} 5} \right\rbrack\end{matrix}$

As apparent from the above Math. 5, an inverse of the coefficientsmatrix is inevitable for channel estimation. This is the reason why thecoefficients matrix must be full rank.

Although any full-rank complex matrix will do, the following twomatrices are typically used in the art because of their simplicity:

Unitary diagonal matrix (exists for any N)

$\begin{matrix}\begin{bmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{bmatrix} & \left\lbrack {{Math}.\mspace{14mu} 6} \right\rbrack\end{matrix}$

Hadamard matrix (exists only for N=2 or a multiple of 4)

$\begin{matrix}\begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {+ 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {- 1} & {+ 1}\end{bmatrix} & \left\lbrack {{Math}.\mspace{14mu} 7} \right\rbrack\end{matrix}$

Physically, using a unitary diagonal matrix means that for antenna nonly the pilots in subset n are non-zero. This means that the receivedvalues at those positions corresponding to pilots of subset n can beused for computing the estimate of channel component h_(n) for thatposition, with no further signal processing.

Using a Hadamard matrix for pilot encoding requires a multiplicationwith this matrix to be performed in the receiver for each cell. Such amultiplication is also referred to as a Hadamard transform. Theschematic of an optimized implementation, known in the art as the fastHadamard transform, is shown in FIG. 13.

The remaining question is how to arrange the scattered pilots in timeand frequency. One option is to keep the same SP pattern like in thesingle-transmitter case, in which case the placement is known and onlythe partitioning into subsets has to be clarified.

Patent Citation 1 provides one possible solution to the question of howto arrange the scattered pilots in time and frequency. According toPatent Citation 1, the scattered pilots are arranged in accordance withthe same pattern as in the single-transmitter case in time andfrequency, and different subsets of pilots are allocated to differentpilot-bearing subcarriers. In other words, the scattered pilots arepartitioned into subsets according to their subcarrier index. Thus, theN subsets of scattered pilots are evenly interleaved in frequency, i.e.along the subcarrier axis. FIGS. 2 and 3 illustrate examples of subsetarrangement patterns as taught by Patent Citation 1 for two and fourtransmitters, respectively.

An alternative approach is known from Patent Citation 2, according towhich the N subsets of scattered pilots are evenly (equally-spaced)interleaved in time, i.e. along the symbol axis. FIGS. 4 and 5illustrate examples of subset arrangement patterns as taught by PatentCitation 2 for two and four transmitters, respectively.

Instead of keeping the pilot pattern of the single-transmitter case,pilots of the N subsets may be grouped, as indicated in FIG. 6. Thisapproach is called “grouped interleaving” and is to be contrasted to theapproach of “equally-spaced interleaving” or “even interleaving”illustrated in FIGS. 2 to 5. In the case of four transmitters (transmitantennas), there may be groups of four pilots (from subsets 1/2/3/4) orgroups of two pilots (from subsets 1/2 and 3/4 for example). FIG. 6shows the former case. Referring to FIG. 6, the groups themselves arearranged so as to be scattered in time and frequency, just like theindividual scattered pilots in the single-transmitter case.

In general, interleaving N pilot subsets in one direction (time orfrequency) increases the effective pilot distance in said direction by afactor of N. In order to compensate for this effect and preserve theeffective distance, the physical pilot distance must be decreased by thesame factor. Thus, if 4 subsets are interleaved in frequency, as inPatent Citation 1, D_(k) must be reduced by a factor of 4. Likewise, ifthe 4 subsets are multiplexed in time, as in Patent Citation 2, D_(s)must be reduced by the same factor.

Since the physical D_(k) and D_(s) must be integers, the effectivedistances D_(k,eff) and D_(s,eff) will always be multiples of 4 wheninterleaving the subsets in one direction only. Such a granularity ofthe subsets may be too coarse for some applications.

CITATION LIST Patent Literature PTL 1:

GB 2449470A

PTL 2:

WO 2009/001528 A1

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an SPpattern with a finer granularity without increasing the total number ofpilots in OFDM systems with a plurality of (e.g., four) transmitantennas. It is also an object of the present invention to provide amethod used by a receiver compatible with multi-antenna transmitters toperform channel estimation in OFDM systems with a plurality of (e.g.,four) transmit antennas.

The above objects are achieved by the features as set forth in theindependent claims.

Preferred embodiments are the subject matter of dependent claims.

It is the particular approach of the present invention to keep the sameSP pattern like in the single-transmitter case, to partition the pilotsinto as many subsets as there are transmit antennas, and to interleavethese subsets both in time and in frequency.

According to a first aspect of the present invention, a multi-antennaOFDM transmitter is provided. The multi-antenna OFDM transmitter has Nantennas, N being an integer greater than or equal to two. Themulti-antenna OFDM transmitter comprises: a multi-antenna encoder forgenerating a plurality of data streams, one for each of the N transmitantennas, each data stream consisting of a succession of OFDM symbols,each OFDM symbol consisting of a plurality of OFDM cells, each OFDM cellbeing associated with one of a plurality of subcarriers; a pilotgeneration unit for generating, for each of the plurality of datastreams, a plurality of scattered pilots, said plurality of scatteredpilots being partitioned into M subsets, each of the scattered pilotsbeing encoded on the basis of the subset to which the scattered pilotbelongs and the data stream into which the scattered pilot is to beinserted, and a plurality of pilot inserting units, each pilot insertingunit for inserting one of the plurality of scattered pilots into acorresponding one of the plurality of data streams in accordance with apredefined periodic pattern in which a temporal spacing between two OFDMsymbols having scattered pilots in OFDM cells associated with the samesubcarrier is equal to D_(s) and a frequency spacing of two subcarriersbearing scattered pilots in any of the OFDM symbols is equal to D_(k),each of D_(s) and D_(k) being an integer greater than or equal to two,wherein M is greater than or equal to N and satisfies a relationshipM=M_(s)M_(k), each of M_(s) and M_(k) being an integer greater than orequal to two, and each of the plurality of pilot inserting units insertsthe scattered pilots in such a manner that a temporal spacing betweentwo OFDM symbols having scattered pilots of the same subset in OFDMcells associated with the same subcarrier is equal to D_(s)M_(s) and afrequency spacing of two subcarriers bearing scattered pilots of thesame subset in any of the OFDM symbols is equal to D_(k)M_(k).

In the above multi-antenna OFDM transmitter, each of M and N may beequal to four, and each of M_(s) and M_(k) may be equal to two.

In the above multi-antenna OFDM transmitter, D_(k) may be equal to two,three, or four.

In the above multi-antenna OFDM transmitter, the pilot generation unitmay encode the scattered pilots by, for each subset, multiplying allscattered pilots of the subset with a constant coefficient that dependson the subset and the data stream into which said all scattered pilotsof the subset are to be inserted.

In the above multi-antenna OFDM transmitter, a matrix formed by theconstant coefficients used for multiplying the scattered pilots may beinvertible, in particular a unitary diagonal matrix or a Hadamardmatrix.

According to a further aspect of the present invention, an OFDM receiveris provided. The OFDM receiver comprises: an OFDM demodulator forobtaining a data stream consisting of a succession of OFDM symbols, eachOFDM symbol consisting of a plurality of OFDM cells, each OFDM cellbeing associated with one of a plurality of subcarriers; a pilotextraction unit for (i) extracting scattered pilots from the data streamin accordance with a predefined periodic pattern in which a temporalspacing between two OFDM symbols having scattered pilots in OFDM cellsassociated with the same subcarrier is equal to D_(s) and a frequencyspacing of two subcarriers bearing scattered pilots in any of the OFDMsymbols is equal to D_(k), each of D_(s) and D_(k) being an integergreater than or equal to two, and (ii) partitioning the extractedscattered pilots into M subsets; and a channel estimation unit forestimating a plurality of channel components from the M subsets ofscattered pilots, each channel component representing a channelcondition between one of a plurality of transmitters and the OFDMreceiver, wherein M satisfies a relationship M=M_(s)M_(k), each of M_(s)and M_(k) being an integer greater than or equal to two, and a temporalspacing between two OFDM symbols having scattered pilots of the samesubset in OFDM cells associated with the same subcarrier is equal toM_(s)D_(s) and a frequency spacing of two subcarriers bearing scatteredpilots of the same subset in any of the OFDM symbols is equal toM_(k)D_(k).

In the above OFDM receiver, M may be equal to four, and each of M_(s)and M_(k) may be equal to two.

In the above OFDM receiver, D_(k) may be equal to two, three, or four.

In the above OFDM receiver, the pilot extraction unit may extract, foreach OFDM symbol, at least one continual pilot from OFDM symbolsassociated with predefined subcarriers and partition the extractedcontinual pilots into the M subsets, and the channel estimation unit mayestimate the plurality of channel components from the M subsets ofscattered pilots and continual pilots.

In the above OFDM receiver, the predefined subcarriers may be thesubcarriers bearing scattered pilots.

In the above OFDM receiver, the predefined subcarriers may be distinctfrom the subcarriers bearing scattered pilots.

In the above OFDM receiver, continual pilots extracted from the samesubcarrier may be partitioned into the same subset.

In the above OFDM receiver, continual pilots extracted from the samesubcarrier may be partitioned into at least two different subsets.

According to a further aspect of the present invention, a method forinserting scattered pilots into transmit signals is provided. The methodis used by a multi-antenna transmitter with N transmit antennas forinserting scattered pilots into transmit signals. The method comprisesthe steps of: generating a plurality of data streams, one for each ofthe N transmit antennas, each data stream consisting of a succession ofOFDM symbols, each OFDM symbol consisting of a plurality of OFDM cells,each OFDM cell being associated with one of a plurality of subcarriers;generating, for each of the plurality of data streams, a plurality ofscattered pilots, said plurality of scattered pilots being partitionedinto M subsets, each of the scattered pilots being encoded on the basisof the subset to which the scattered pilot belongs and the data streaminto which the scattered pilot is to be inserted, and inserting one ofthe plurality of scattered pilots into a corresponding one of theplurality of data streams in accordance with a predefined periodicpattern in which a temporal spacing between two OFDM symbols havingscattered pilots in OFDM cells associated with the same subcarrier isequal to D_(s) and a frequency spacing of two subcarriers bearingscattered pilots in any of the OFDM symbols is equal to D_(k), each ofD_(s) and D_(k) being an integer greater than or equal to two, wherein Mis greater than or equal to N and satisfies a relationship M=M_(s)M_(k),each of M_(s) and M_(k) being an integer greater than or equal to two,and in the inserting step, the scattered pilots are inserted in such amanner that a temporal spacing between two OFDM symbols having scatteredpilots of the same subset in OFDM cells associated with the samesubcarrier is equal to D_(s)M_(s) and a frequency spacing of twosubcarriers bearing scattered pilots of the same subset in any of theOFDM symbols is equal to D_(k)M_(k).

In the above method, each of M and N may be equal to four, and each ofM_(s) and M_(k) may be equal to two.

According to a further aspect of the present invention, a method forestimating, at an OFDM receiver, channel properties between the OFDMreceiver and each of N transmit antennas is provided. The methodcomprises the steps of: obtaining a data stream consisting of asuccession of OFDM symbols, each OFDM symbol consisting of a pluralityof OFDM cells, each OFDM cell being associated with one of a pluralityof subcarriers; extracting scattered pilots from the data stream inaccordance with a predefined periodic pattern in which a temporalspacing between two OFDM symbols having scattered pilots in OFDM cellsassociated with the same subcarrier is equal to D_(s) and a frequencyspacing of two subcarriers bearing scattered pilots in any of the OFDMsymbols is equal to D_(k), each of D_(s) and D_(k) being an integergreater than or equal to two, and partitioning the extracted scatteredpilots into M subsets; and estimating a plurality of channel componentsfrom the M subsets of scattered pilots, each channel componentrepresenting a channel condition between one of a plurality oftransmitters and the OFDM receiver, wherein M satisfies a relationshipM=M_(s)M_(k), each of M_(s) and M_(k) being an integer greater than orequal to two, and a temporal spacing between two OFDM symbols havingscattered pilots of the same subset in OFDM cells associated with thesame subcarrier is equal to M_(s)D_(s) and a frequency spacing of twosubcarriers bearing scattered pilots of the same subset in any of theOFDM symbols is equal to M_(k)D_(k).

In the above method, each of M and N may be equal to four, and each ofM_(s) and M_(k) may be equal to two.

The above and other objects and features of the present invention willbecome more apparent from the following description and preferredembodiments given in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional single-transmitter SP pattern as it is usedin the European digital broadcasting standard DVB-T.

FIG. 2 shows a conventional two-transmitter (two-transmit antenna) SPpattern with equally-spaced interleaving in frequency.

FIG. 3 shows a conventional four-transmitter (four-transmit antenna) SPpattern with equally-spaced interleaving in frequency.

FIG. 4 shows a conventional two-transmitter (two-transmit antenna) SPpattern with equally-spaced interleaving in time.

FIG. 5 shows a conventional four-transmitter (four-transmit antenna) SPpattern with equally-spaced interleaving in time.

FIG. 6 shows a conventional SP pattern for four transmitters (transmitantennas) and grouped interleaving in time and frequency.

FIG. 7 shows an SP pattern for four transmitters (transmit antennas) andequally-spaced interleaving in time and frequency in accordance with anembodiment of the present invention.

FIG. 8 shows another SP pattern for four transmitters (transmitantennas) and equally-spaced interleaving in time and frequency inaccordance with an embodiment of the present invention.

FIG. 9 illustrates the channel estimation process for thesingle-transmitter case, using separable interpolation in time andfrequency.

FIG. 10 shows the receivers, transmitters, and the eight channelcomponents in a 4×2 MIMO configuration.

FIG. 11 shows an exemplary block diagram of a multi-antenna OFDMtransmitter.

FIG. 12A shows the 16 pilot-multiplication coefficients for the4-transmitter case.

FIG. 12B shows the preferred realization of FIG. 12A as a Hadamardmatrix.

FIG. 13 illustrates the fast Hadamard transform.

FIG. 14 is an illustration of the channel estimation process in areceiver for 4-transmitter OFDM.

FIG. 15 shows a block diagram of a receiver for 4-transmitter OFDM,corresponding to the process of FIG. 14.

FIG. 16 shows a receiver configuration for 4×2 MIMO, where each OFDMreceiver provides its own channel estimates to the MIMO decoding stage.

FIG. 17 shows an example of an inventive SP pattern with additionalcontinual pilots located on SP-bearing subcarriers.

FIG. 18 shows another example of an inventive SP pattern with additionalcontinual pilots located on non-SP-bearing subcarriers.

FIG. 19 shows an inventive SP pattern of signals transmitted from thefirst antenna and coefficients by which the SPs are multiplied.

FIG. 20 shows an inventive SP pattern of signals transmitted from thesecond antenna and coefficients by which the SPs are multiplied.

FIG. 21 shows an inventive SP pattern of signals transmitted from thethird antenna and coefficients by which the SPs are multiplied.

FIG. 22 shows an inventive SP pattern of signals transmitted from thefourth antenna and coefficients by which the SPs are multiplied.

FIG. 23 shows an inventive SP pattern where there are six subsets, withequally-spaced interleaving in both time and frequency.

FIG. 24 shows an inventive SP pattern where there are six subsets, withequally-spaced interleaving in both time and frequency.

FIG. 25 shows an inventive SP pattern where there are six subsets, withequally-spaced interleaving in both time and frequency.

FIG. 26 shows an inventive SP pattern where there are six subsets, withequally-spaced interleaving in both time and frequency.

FIG. 27 shows an inventive SP pattern for each subset, which is distinctfrom the SP pattern shown in FIG. 1.

FIG. 28 shows an exemplary configuration of a digital broadcast systempertaining to an embodiment of the present invention.

FIG. 29 is a functional structural diagram showing an exemplarystructure of a receiver pertaining to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods for inserting scattered pilots(SPs) into the transmit signals of multi-antenna OFDM systems, forestimating channel properties on the basis of the scattered pilots, amulti-antenna OFDM transmitter, and a corresponding OFDM receiver. Theinventive methods and apparatuses achieve a high granularity of thescattered pilots in both the time and the frequency directions, which isa prerequisite for efficiently estimating channel properties inapparatuses that receive signals into which scattered pilots have beeninserted.

To this end, it is the particular approach of the present invention tokeep the same scattered pilot pattern like in the single-transmittercase, to partition the pilots into as many subsets as there are transmitantennas, and to interleave these subsets both in time and in frequency.In this manner, the granularity of the subsets is reduced compared toconventional technologies since, in the four transmitter case, D_(k,eff)and D_(s,eff) are multiples of 2 instead of 4. This offers increasedflexibility in designing the scattered pilot patterns.

FIG. 7 shows a scattered pilot pattern for four transmitters andequally-spaced interleaving in time and frequency in accordance with anembodiment of the present invention. Each circle represents one OFDMcell, each row of circles corresponds to one OFDM symbol, and eachcolumn represents one subcarrier. Pilots are indicated by large circles,whereas data cells are indicated by small circles.

The scattered pilots are divided into four subsets denoted by numerals1, 2, 3, and 4, respectively. The number of subsets corresponds to thenumber of distinct transmitter antennas, i.e., four in this case.

As it is apparent from a comparison of FIG. 1 and FIG. 7, the scatteredpilots are arranged in accordance with the same pattern as in theconventional single-transmitter case (DVB-T), i.e., in the form of adiagonal grid. The pilot cells, whatever subset they belong to, areinterleaved with the data cells such that the distance between twoadjacent pilot cells in a pilot-bearing subcarrier in the time directionis D_(s)=4 and the distance between two adjacent SP-bearing subcarriersin the frequency direction is D_(k)=3.

The pilot cells of any given subset, on the other hand, are interleavedwith the pilot cells of the other subsets both in time and frequency. Inother words, the subset to which a pilot belongs alternates both in thefrequency and the time directions. This is to be contrasted with theconventional multi-antenna SP patterns of FIGS. 2 and 3, wherein acertain subcarrier or a certain OFDM symbol carries only pilots of oneand the same subset.

With the inventive SP pattern of FIG. 7, the effective distance in thetime direction D_(s,eff) between two pilots of the same subset equals2D_(s), i.e., twice the SP spacing in the time direction D_(s), and theeffective distance in the frequency direction D_(k,eff) between twopilots of the same subset equals 2D_(k), i.e., twice the SP spacing inthe frequency direction D_(k).

This is to be contrasted with the conventional multi-antenna SP patternsof FIG. 3, wherein the effective distance in the frequency directionD_(k,eff) between two pilots of the same subset equals 4D_(k), i.e.,four times the SP spacing in the frequency direction D_(k). This is alsoto be contrasted with the conventional multi-antenna SP patterns of FIG.5, wherein the effective distance in the time direction D_(s,eff)between two pilots of the same subset equals 4D_(s), i.e., four timesthe SP spacing in the time direction D_(s).

Hence, the inventive SP pattern of FIG. 7 provides a finer granularityin the distribution of scattered pilots of the same subset than theconventional SP patterns shown in FIGS. 3 and 5.

FIG. 8 shows another SP pattern for four transmitters and equally-spacedinterleaving in time and frequency in accordance with another embodimentof the present invention. In this SP pattern, similar granularity ismaintained both in time and frequency. This SP pattern is in manyaspects similar to that of FIG. 7 and achieves the same advantages. Arepetition of the detailed explanations provided in connection with FIG.7 is therefore omitted.

The advantages of the inventive SP patterns become even more apparent bycomparing “unit cells” of the SP lattices. A unit cell, originallydefined in the context of crystallography, is the smallest unit of alattice from which the entire (periodic) lattice may be reconstructed bymeans of translations only. Unit cells of the SP patterns are indicatedby dashed lines in FIGS. 2 to 8. Obviously, the unit cells of the SPpatterns shown in FIGS. 7 and 8 are more compact than those of FIGS. 3and 5, which are more extensive in either the frequency (FIG. 3) or thetime direction (FIG. 5).

FIG. 11 shows a block diagram for a multi-antenna OFDM transmitter usingscattered pilots. The bits to be transmitted are fed at the input of anencoder 1110. In the encoder 1110, these bits undergo a BICM(bit-interleaved coding and modulation) encoding, which produces complexsymbols at its output. The BICM encoding consists of three basicsteps: 1) FEC (forward error correction) encoding, 2) bit interleaving,and 3) modulation. Such a process is well known in the art. The FEC codeis typically an LDPC (low-density parity check) code or a Turbo code,and the modulation is typically QAM (quadrature amplitude modulation).

The complex symbols produced by the BICM encoder 1110 are fed to amulti-antenna processor 1120, wherein they undergo a multi-antennaencoding, whereby the input stream is encoded to generate multipleparallel streams of equal data rate, one for each transmitter/antenna.The output streams carry exactly the same information as the inputstreams. Typically an STBC (space-time block code) is used for thispurpose. There are many STBC variants known in the art.

The complex symbols produced by the STBC encoding are then mapped ontothe time-frequency OFDM grid, according to a specific algorithm, whichdoes not make the object of the present invention. The output of themapping process consists of OFDM symbols, which in turn consist ofcomplex OFDM cells. The cells are then interleaved by a set of symbolinterleavers (1130-1, 1130-2, 1130-3, 1130-4) in order to improve thefrequency diversity of the encoded data. Such interleaving is well knownin the art and is also referred to as frequency interleaving since theOFDM symbols span the available frequency bandwidth. The mapping andinterleaving are identical for all transmitters/antennas.

Following the OFDM symbol interleaving, the scattered pilots (SP) aregenerated by pilot generation unit 1140 and inserted by a set of pilotinsertion units (1150-1, 1150-2, 1150-3, 1150-4). The SPs are not thesame for all transmitters/antennas. Each of the pilot insertion unitsinserts the input SPs into the OFDM signals so as to achieve, forexample, the symbol patterns shown in FIGS. 7 and 8. In order to be ableto estimate the individual channel components, the SPs are partitionedinto as many subsets as there are transmitters/antennas. All pilotsbelonging to a subset are multiplied by a constant coefficient thatdepends on the subset and the transmitter (transmit antenna). In thefour-transmitter case, there are 16 coefficients, which can be expressedas a 4×4 matrix, as shown in FIGS. 12A and 12B. The rows correspond tothe transmitters (transmit antennas) and the columns to the SP subsets.

The resulting OFDM symbols including data cells and pilot cells are thenfed to the OFDM modulators (1160-1, 1160-2, 1160-3, 1160-4), followed byup-converters (1170-1, 1170-2, 1170-3, 1170-4), RF-amplifiers (1180-1,1180-2, 1180-3, 1180-4), and finally transmitted via transmit antennas.

On the receiver side, the channel estimation process is similar to thatused in the single-transmitter case in FIG. 9. Instead of one channelestimation process, however, four will be performed in parallel, one foreach SP subset, as shown in FIG. 14. If a pilot encoding is used, e.g.Hadamard, an additional processing step, e.g. Hadamard transform, isrequired in order to separate the four channel components. Thisprocessing step is performed for each OFDM cell independently.

A possible receiver block diagram for the OFDM-specific part is shown inFIG. 15. A pilot extraction unit 1540, 2D interpolation units (1550-1,1550-2, 1550-3, 1550-4), and a Hadamard transform unit 1560 shown inFIG. 15 perform the process of FIG. 14.

An RF-frontend 1510 receives an RF signal, which is fed to a downconverter 1520. The down converter 1520 performs downconversion on theRF signal, which is then fed to OFDM demodulator 1530. This RF signalfed from the down converter 1520 is demodulated by the OFDM demodulator1530. From the demodulated signal, the pilots are extracted by pilotextraction unit 1540. The pilot extraction unit 1540 is adapted forpartitioning the extracted pilots into subsets in accordance with theinventive SP patterns described above. Signals other than the pilots areoutput as data signals. The pilots of each of these subsets are then fedto a corresponding one of a set of the 2D interpolation units (1550-1,1550-2, 1550-3, 1550-4). It is to be noted that the two distinctinterpolation steps are merged into a single block “2-D interpolation”.The signals obtained through the interpolation performed by the 2Dinterpolation units (1550-1, 1550-2, 1550-3, 1550-4) are fed to thetransform unit 1560 and converted into channels. Depending on how thepilots of different subsets are encoded, a Hadamard transform is appliedin the transform unit 1560 in order to extract the channel estimates h₁,. . . h₄. A delay compensation unit 1580 is provided on the data path,and compensates for a group delay introduced by the interpolationprocess based on the pilots in each subset. By thus compensating for adata delay, the delay compensation unit 1580 realigns data and channelsin each of symbol deinterleavers (1570-1, 1570-2, 1570-3, 1570-4).

The MIMO and BICM decoding are not OFDM specific, but require the dataand the associated channel estimates from all receivers/antennas. Theactual decoding architecture strongly depends on the particular STBC, aswell as on the desired reception performance. Optimal results areobtained when the complex symbols encoded in an STBC block are decodedand demodulated jointly. FIG. 16 shows an exemplary block diagramcontaining two OFDM receivers 1610-1 and 1610-2 feeding a common MIMOand BICM decoding stage 1620.

According to a further aspect of the present invention, modifiedcontinual pilots (CP) are inserted into the SP pattern. Conventional CPsare pilots that are present in every symbol on a given subcarrier. Theycan be located on an SP-bearing subcarrier or on a non-SP-bearingsubcarrier and are generally not subject to any additional processing.According to the present invention, however, the CPs are alsopartitioned into subsets like the SPs.

FIG. 17 shows an example of an inventive SP pattern with additionalcontinual pilots located on SP-bearing subcarriers. The CPs on a givenSP-bearing subcarrier are partitioned into the two subsets the SPs ofthat subcarrier belong to. In the figure, the continual pilots arerepresented by rectangles and scattered pilots by large circles. OFDMcells that are used both for scattered and continual pilots arerepresented by a combination of a circle and a rectangle. Numeralsindicate the corresponding subset.

The CP partitioning is performed so that those CPs that are also SPswill not change their subset. Moreover, the CP partitioning must bebalanced between the two subsets, and the number of transitions insubsets to which the CPs belong must be minimized along the time axis(symbol direction). These constraints lead to a partitioning consistingof alternating contiguous groups of D_(s) CPs, as in FIG. 17(subcarriers 6, 9, 24, 27). The main feature is that, with respect tothe CPs, there is a subset change every D_(s) pilots. The locationswhere the subsets of the CPs change are not limited to the ones shown inFIG. 17. There are D_(s) possibilities to choose from as candidates forsuch positions.

FIG. 18 shows, in a manner similar to FIG. 17, an example of aninventive SP pattern with additional continual pilots located onnon-SP-bearing subcarriers.

For the CPs on non-SP-bearing subcarriers there are two possibilities.First, all CPs on the same subcarrier may be kept in one subset, as itis illustrated in FIG. 18 for subcarriers 5, 7, 14, and 16. Second, twosubsets of pilots may be alternated on each subcarrier in a mannersimilar to that for SP-bearing subcarriers, as it is illustrated in FIG.18 for subcarriers 23, 28, 31, and 34. Preferably, the CPs are evenlydistributed among the four subsets.

Summarizing, the present invention relates to orthogonalfrequency-division multiplexing (OFDM) communication systems withmultiple (e.g., four) transmit antennas and one or morereceivers/antennas for transmitting and receiving OFDM signals, and inparticular to methods for inserting scattered pilots (SPs) into thetransmit signals of such OFDM systems, for estimating channel propertieson the basis of the scattered pilots, a multi-antenna OFDM transmitter,and an OFDM receiver. In this context, it is the particular approach ofthe present invention to keep the same SP pattern like in thesingle-transmitter case, to partition the pilots into as many subsets asthere are transmit antennas, and to interleave these subsets both intime and in frequency. In this manner, the granularity of pilots of thesame subset is reduced. This offers increased flexibility in designingthe scattered pilot patterns and greater accuracy of the estimatedchannel properties.

Supplementary Notes

The implementation methods pertaining to the present invention are notlimited to those described in the above embodiments. The followingexplains variations of conceptions of the present invention.

(1) The above embodiments have not provided detailed descriptions of thesignals transmitted by each antenna. Described below are SPs in thesignals transmitted by each antenna.

FIGS. 19 to 22 each show the SP pattern of the signals transmitted byeach antenna in the case of four-antenna transmission. Here, in encodingthe SPs, the SPs are multiplexed by the coefficients shown in the matrixof FIG. 12A.

FIG. 19 shows a symbol pattern of the signals transmitted by a firstantenna (so named for convenience), which is one of the four antennas.As is apparent from the comparison between FIG. 19 and FIG. 7, whichshows the symbol pattern pertaining to the present invention, the SPsbelonging to the subsets indicated by the numbers 1, 2, 3 and 4 in FIG.7 are respectively multiplied by the coefficients C₁₁, C₁₂, C₁₃ and C₁₄corresponding to the first antenna as shown in FIG. 12A.

Meanwhile, FIGS. 20, 21 and 22 respectively show symbol patterns of theOFDM signals to be transmitted by the second antenna, the third antennaand the fourth antenna.

As is apparent from the comparison between FIGS. 19 and 20, SPs that aremultiplied by the coefficient C₁₂ in the signals transmitted by thesecond antenna are located in the positions of SPs that are multipliedby the coefficient C₁₁ in the signals transmitted by the first antenna.Pilots that are multiplied by the coefficients C₁₃ and C₁₄ in thesignals transmitted by the third and fourth antennas (respectively shownin FIGS. 21 and 22) are also located in such positions.

As indicated by the signals shown in FIGS. 19 to 22, which arerespectively transmitted by the four antennas, the SPs included in thesignals correspond to a different one of the antennas, and areperiodically arranged in such a manner that two adjacent SPs belongingto one subset are spaced with an SP belonging to another subset arrangedtherebetween, both in time and in frequency.

(2) The above embodiments have described the case where the number ofsubsets is four. However, the number of subsets is not limited to four,but may be any number that is a product of numbers M_(s) (an integergreater than or equal to two) and M_(k) (an integer greater than orequal to two).

At this time, the distance between two adjacent pilots belonging to thesame subset in one subcarrier is D_(s)×M_(s), based on D_(s) and M_(s)described in the above embodiments. The distance between two adjacentsubcarriers including pilots belonging to the same subset isD_(k)×M_(k).

As one example, M_(s)=2 and M_(k)=3. In this case, the SP pattern is thesame as, for instance, the one shown in FIG. 23. In other words, SPsbelonging to each subset should be arranged as shown in FIG. 23.

As another example, in a case where M_(s)=2 and M_(k)=3, SPs belongingto each subset may be arranged as shown in, for example, FIGS. 24 to 26.In this case, it is preferable that information indicating which one ofthe SP patterns should be used be either preset in each receiver, ornotified to each receiver by the transmitters. FIGS. 25 and 26 showexamples of an SP pattern with M_(s)=3 and M_(k)=2.

(3) In the above embodiments, methods of encoding the SPs using theHadamard transform, as shown in FIG. 12, have been described. However,the SPs may be encoded using any orthogonal transform method with use ofa unitary diagonal matrix as has been described in Background Art, theFourier transform matrix shown in the following Math. 8, and the like.It should be noted that since an inverse of the orthogonal transformmatrix must exist, the orthogonal transform matrix must be full rank.

$\begin{matrix}\begin{bmatrix}e^{j\; 2\pi \frac{0}{4}} & e^{j\; 2\pi \frac{0}{4}} & e^{j\; 2\pi \frac{0}{4}} & e^{j\; 2\pi \frac{0}{4}} \\e^{j\; 2\pi \frac{0}{4}} & e^{j\; 2\pi \frac{1}{4}} & e^{j\; 2\pi \frac{2}{4}} & e^{j\; 2\pi \frac{3}{4}} \\e^{j\; 2\pi \frac{0}{4}} & e^{j\; 2\pi \frac{2}{4}} & e^{j\; 2\pi \frac{4}{4}} & e^{j\; 2\pi \frac{6}{4}} \\e^{j\; 2\pi \frac{0}{4}} & e^{j\; 2\pi \frac{3}{4}} & e^{j\; 2\pi \frac{6}{4}} & e^{j\; 2\pi \frac{9}{4}}\end{bmatrix} & \left\lbrack {{Math}.\mspace{14mu} 8} \right\rbrack\end{matrix}$

In a case where SPs to be transmitted by N transmit antennas are encodedafter being partitioned into N subsets, the Fourier transform matrixshown in Math. 9 may be used.

$\begin{matrix}\begin{bmatrix}e^{j\; 2\pi \frac{0}{N}} & e^{j\; 2\pi \frac{0}{N}} & \ldots & e^{j\; 2\pi \frac{0}{N}} \\e^{j\; 2\pi \frac{0}{N}} & e^{j\; 2\pi \frac{1}{N}} & \ldots & e^{j\; 2\pi \frac{N - 1}{N}} \\\vdots & \vdots & \ddots & \vdots \\e^{j\; 2\pi \frac{0}{N}} & e^{j\; 2\pi \frac{N - 1}{N}} & \ldots & e^{j\; 2\pi \frac{{({N - 1})}{({N - 1})}}{N}}\end{bmatrix} & \left\lbrack {{Math}.\mspace{14mu} 9} \right\rbrack\end{matrix}$

(4) In the above embodiments, the number of the transmitters (transmitantennas) has been described as the same as the number of subsets, i.e.,four. It should be noted, however, that the number of the transmitters(transmit antennas) may be smaller than or equal to the number of thesubsets.

As one example, the number of subsets and the number of transmitters(transmit antennas) may be four and three, respectively. As anotherexample, the number of subsets and the number of transmitters (transmitantennas) may be six and five, respectively. That is to say, the numberof subsets and the number of transmitters (transmit antennas) may bearbitrary, as long as each receiver can distinguish the signalstransmitted by the respective transmitters (transmit antennas).

For instance, in a case where the number of subsets and the number oftransmitters (transmit antennas) are four and three, respectively,provided that the matrix shown in FIG. 12A is used, the coefficients ina column corresponding to one of the antennas are not used, and threeother antennas transmit the OFDM signals in which are arranged SPsmultiplied by the coefficients in the columns corresponding to thesethree other antennas. For example, assume a case where the fourthantenna shown in FIG. 12A does not exist. In this case, (i) the firstantenna transmits the OFDM signals in which are arranged SPs multipliedby the coefficients C₁₁, C₂₁, C₃₁ and C₄₁, (ii) the second antennatransmits the OFDM signals in which are arranged SPs multiplied by thecoefficients C₁₂, C₂₂, C₃₂ and C₄₂, and (iii) the third antennatransmits the OFDM signals in which are arranged SPs multiplied by thecoefficients C₁₃, C₂₃, C₃₃ and C₄₃. Here, none of the transmitters(transmit antennas) transmits the OFDM signals in which are arranged SPsmultiplied by the coefficients C₁₄, C₂₄, C₃₄ and C₄₄. Each receiver thathas received the OFDM signals transmitted in the above manner performsdemodulation by estimating channels between the three transmit antennasbased on the SPs belonging to the four subsets. In this case, out of allthe coefficients shown in the matrix of FIG. 12A, the coefficients inthe column corresponding to the fourth antenna need not be used.

In the above (3) of Supplementary Notes, the number of subsets isdescribed as a product of numbers M_(s) (an integer greater than orequal to two) and M_(k) (an integer greater than or equal to two). Putanother way, the number of subsets may be a composite number that isgreater than or equal to the number of transmitters.

(5) The SP pattern shown in FIG. 1 pertaining to the above embodimentsis merely an example. For example, in DVB-T2, other SP patterns arepermitted, one example of which is an SP pattern where SPs are insertedinto every sixth subcarrier as shown in FIG. 27. An SP pattern similarto that of FIG. 27 may be employed in transmitters of the presentinvention. In this case, the SPs should be inserted according to the SPinsertion methods described in the above embodiments to realize thepattern of pilots belonging to each subset as shown in FIG. 27. That isto say, provided that the distance between two adjacent SP symbols in anSP-bearing subcarrier is D_(s) and the distance between two adjacentSP-bearing subcarriers is D_(k), (i) the distance between SPs belongingto the same subset should be greater than or equal to 2D_(s) in onesubcarrier along the symbol direction, (ii) the distance betweensubcarriers bearing SPs belonging to the same subset should be greaterthan or equal to 2D_(k), and (iii) between two SPs belonging to acertain subset, another SP belonging to a subset other than said certainsubset is arranged, both in time and frequency. In the example of FIG.27, D_(s)=2 and D_(k)=6.

(6) The above embodiments and variations may be combined partially.

(7) The communication systems pertaining to the present invention with aplurality of transmitters and one or more receivers may each be the MIMOsystem or the MISO system, as long as each transmitter (transmitantenna) is configured to transmit the signals in which are arranged SPsbelonging to subsets corresponding to the respective transmitters (see,for example, FIG. 7).

(8) It is possible to provide a control program that is composed ofprogramming codes such as machine language and high-level language andthat causes a processor in each transmitter, or various circuitsconnected to that processor, to execute the process for inserting pilotsinto the OFDM transmit signals described in the above embodiments. Sucha control program may be recorded on a recording medium, or may bedistributed/disseminated via various types of communication chancels.Furthermore, it is also possible to provide a control program that iscomposed of programming codes such as machine language and high-levellanguage and that causes a processor in each transmitter, or variouscircuits connected to that processor, to execute the process forestimating channel properties of the OFDM reception signals described inthe above embodiments. Such a control program may be recorded on arecording medium, or may be distributed/disseminated via various typesof communication chancels. Examples of such a recording medium includean IC card, a hard disk, an optical disc, a flexible disk, ROM, andflash memory. The distributed/disseminated control program is stored inmemory or the like that can be read by the processor so as to beprovided for use. Each of the functions described in the aboveembodiments can be realized by the processor executing the controlprogram. The processor may directly execute the control program, orexecute the control program after compiling the same, or execute thecontrol program with an interpreter.

(9) Each of the functional constituent elements included in eachtransmitter and each receiver described in the above embodiments (theHadamard transform unit, the pilot generation unit, etc.) may berealized as a circuit for executing its functions, or may be realized byone or more processors executing a program, or may be configured as apackaged integrated circuit such as an IC and an LSI. Such a packagedintegrated circuit is built in each device to be provided for use. Thisway, each device can realize the functions described in the aboveembodiments.

(10) The following describes an exemplary application of thetransmission/reception methods explained in the above embodiments, aswell as an exemplary configuration of a system using suchtransmission/reception methods.

FIG. 28 shows an exemplary configuration of a system including devicesthat perform the transmission/reception methods explained in the aboveembodiments. The transmission/reception methods explained in the aboveembodiments are implemented in a digital broadcast (or communication)system 2800 shown in FIG. 28 that includes a broadcast station (or abase station) 2801 and various types of receivers, such as a TV(television) 2811, a DVD recorder 2812, an STB (Set Top Box) 2813, acomputer 2820, an in-vehicle TV 2841, and a cell phone 2830. Morespecifically, the broadcast station (base station) 2801 transmits atransmission data stream (e.g., a multiplexed data stream obtained bymultiplexing a video data stream, an audio data stream, etc.) to apredetermined transmission band by using the transmission methodsexplained in the above embodiments.

The signals transmitted from the broadcast station (base station) 2801are received by antennas (e.g., antennas 2810 and 2840) that are eitherbuilt in the receivers, or positioned outside the receivers while beingconnected to the receivers. Each receiver performs the receptionoperations explained in the above embodiments on the signals received byits antenna, and obtains the received data stream. This way, the digitalbroadcast system 2800 can achieve the effects of the present inventiondescribed in the above embodiments.

The video data stream included in the multiplexed data stream has beenencoded with a video encoding method conforming to such standards asMPEG (Moving Picture Experts Group)-2, MPEG-4 AVC (Advanced VideoCoding), and VC-1. The audio data stream included in the multiplexeddata stream has been encoded with an audio encoding method such as DolbyAC (Audio Coding)-3, Dolby Digital Plus, MLP (Meridian LosslessPacking), DTS (Digital Theater Systems), DTS-HD, and linear PCM(Pulse-Code Modulation).

FIG. 29 shows the structure of a receiver 2900 used in a digitalbroadcast system, as one example of devices that perform the receptionmethods explained in the above embodiments. As shown in FIG. 29, oneexemplary structure of the receiver 2900 is such that a modem portion isconstituted by one LSI (or one chipset), and a codec portion isconstituted by another LSI (or another chipset). The structure of thereceiver 2900 shown in FIG. 29 is equivalent to that of such devices asthe TV (television) 2811, the DVD recorder 2812, the STB (Set Top Box)2813, the computer 2820, the in-vehicle TV 2841, and the cell phone 2830shown in FIG. 28. The receiver 2900 includes a tuner 2901 and ademodulation unit 2902. The tuner 2901 converts radio frequency signalsreceived by an antenna 2960 into baseband signals. The demodulation unit2902 obtains a multiplexed data stream by performing the receptionoperations explained in the above embodiments on the baseband signals.As a result, the effects of the present invention described in the aboveembodiments can be achieved.

The receiver 2900 also includes a stream input/output unit 2903, asignal processing unit 2904, an AV (Audio and Visual) output unit 2905,an audio output unit 2906, and a video display unit 2907. The streaminput/output unit 2903 demultiplexes the video data stream and the audiodata stream from the multiplexed data stream obtained by thedemodulation unit 2902. The signal processing unit 2904 decodes thevideo data stream into a video signal by using a video decoding methodcorresponding to the demultiplexed video data stream, and decodes theaudio data into an audio signal by using an audio decoding methodcorresponding to the demultiplexed audio data stream. The AV output unit2905 outputs the decoded audio signal to the audio output unit 2906, andoutputs the decoded video signal to the video display unit 2907.Alternatively, the AV output unit 2905 outputs the decoded audio andvideo signals to an AV (Audio and Visual) output IF (interface) 2911.The audio output unit 2906 (e.g., a speaker) outputs the decoded audiosignal. The video display unit 2907 (e.g., a display) displays thedecoded video signal.

By way of example, a user transmits information on a selected channel (aselected (TV) program, selected audio broadcasting, etc.) to anoperation input unit 2910 with use of a remote controller 2950.Thereafter, the receiver 2900 obtains the multiplexed data streamcorresponding to the selected channel by performing demodulation, errorcorrection decoding, and the like on the received signals thatcorrespond to the selected channel. At this time, the receiver 2900selects a reception method being appropriate to the selected channelaccording to information of a transmission method (for example, the SPpatterns, the number of subsets, and a modulation method and an errorcorrection method performed on the data stream transmitted by the datacells, which are discussed in the above embodiments) obtained fromcontrol symbols included in the received signals. This way, the receiver2900 can obtain data contained in the data cells transmitted from thebroadcast station (base station). In the example described above, theuser selects a channel with use of the remote controller 2950. However,the above-described operations are performed also when a channel isselected with use of a channel selection key built in the receiver 2900.

With the above structure, the user can view the program that has beenreceived by the receiver 2900 using the reception methods described inthe above embodiments.

Assume a case where the above-described receiver 2900 pertaining to thepresent invention is built in a TV, a recording device (e.g., a DVDrecorder, a Blu-ray recorder, an HDD recorder, and an SD card), and acell phone. In this case, if the multiplexed data obtained throughdemodulation and error correction decoding by the demodulation unit 2902includes (i) data for correcting a default (bug) in software used tocause the TV and the recording device to operate, or (ii) data forcorrecting a default (bug) in software used to prevent leaks of personalinformation and recorded data, then a default in software provided inthe TV and the recording device may be corrected by installing suchdata. If the multiplexed data obtained through demodulation and errorcorrection decoding by the demodulation unit 2902 includes data forcorrecting a default (bug) in software provided in the receiver 2900,then a default in the receiver 2900 may be corrected with such data.This way, the TV, the recording device and the cellular phone in whichthe receiver 2900 is built can operate in a more stable manner.

The receiver 2900 of the present embodiment also includes a recordingunit (drive) 2908 that records the following (i) through (iii) on arecording medium such as a magnetic disk, an optical disc and anonvolatile semiconductor memory: (i) part of the multiplexed datastream obtained through demodulation and error correction decoding bythe demodulation unit 2902 (In some occasions, error correction decodingis not performed on signals obtained through demodulation by thedemodulation unit 2902. Also, the receiver 2900 may perform other signalprocessing after error correction decoding. These are true of thefollowing descriptions that use similar expressions about errorcorrection decoding to those used in this section.); (ii) datacorresponding to the data of (i), such as data obtained by compressingthe data of (i); and (iii) data obtained by processing video and audio.Here, the optical disc is, for example, a recording medium on/from whichinformation is recorded/read using laser light, such as DVD (DigitalVersatile Disc) and BD (Blu-ray Disc). The magnetic disk is, forexample, a recording medium that stores information by magnetizing amagnetic material using magnetic flux, such as FD (Floppy Disk)(registered trademark) and a hard disk. The nonvolatile semiconductormemory is, for example, a recording medium made up of semiconductorelements, such as flash memory and ferroelectric random access memory.Examples of the nonvolatile semiconductor memory include an SD card anda flash SSD (Solid State Drive) that incorporate flash memory. It shouldbe noted that the above-listed types of recording media are merelyexamples. It goes without saying that the recording may be performedwith use of a recording medium other than the above-listed recordingmedia.

With the above structure, the user can record and store the program thathas been received by the receiver 2900 by using the reception methodsdescribed in the above embodiments. Accordingly, the receiver 2900 canread the recorded data and the user can view the program correspondingto the recorded data at any time after the broadcast time of theprogram.

It has been described above that in the receiver 2900, the recordingunit 2908 records the multiplexed data stream obtained throughdemodulation and error correction decoding by the demodulation unit2902. Alternatively, part of the multiplexed data stream may beextracted and recorded. For instance, when the multiplexed data streamobtained by the demodulation unit 2902 includes contents etc. that areprovided from data broadcast services and that are different from thevideo data stream and the audio data stream, the recording unit 2908 mayrecord a new multiplexed data stream that is obtained by extracting andmultiplexing the video data stream and the audio data stream in themultiplexed data stream obtained by the demodulation unit 2902.Alternatively, the recording unit 2908 may record a new multiplexed datastream that is obtained by multiplexing one of the video data stream andthe audio data stream included in the multiplexed data stream obtainedby the demodulation unit 2902. In addition, the recording unit 2908 mayrecord the aforementioned contents that are included in the multiplexeddata and that are provided from the data broadcast services.

As one example, the stream input/output unit 2903 performs theprocessing for extracting and multiplexing part of plural data piecesincluded in the multiplexed data obtained through demodulation and errorcorrection decoding by the demodulation unit 2902. More specifically,with an instruction from a controller such as CPU (not illustrated), thestream input/output unit 2903 generates a new multiplexed data stream by(i) demultiplexing the multiplexed data stream obtained by thedemodulation unit 2902 into demultiplexed data streams, such as a videodata stream, an audio data stream, and other contents provided from databroadcast services, and (ii) extracting and multiplexing only aspecified data stream out of the demultiplexed data streams. Which datastream should be extracted from the demultiplexed data streams may bedetermined by the user, or may be predetermined for each type ofrecording media.

With the above structure, the receiver 2900 can extract and record onlythe data required to view the recorded program. This can reduce the datasize of the data to be recorded.

It has been described above that the recording unit 2908 records themultiplexed data obtained through demodulation and error correctiondecoding by the demodulation unit 2902. Alternatively, the recordingunit 2908 may perform the recording in the following steps: (i)converting the original video data stream included in the multiplexeddata stream obtained by the demodulation unit 2902 into a new video datastream, which has been encoded with a video encoding method that isdifferent from a video encoding method performed on the original videodata stream, so that the data size or bit rate of the new video datastream is smaller/lower than the data size or bit rate of the originalvideo data stream; and (ii) recording a new multiplexed data streamobtained by multiplexing the post-conversion new video data stream. Thevideo encoding methods that are respectively performed on the originalvideo data stream and the post-conversion new video data stream mayconform to different standards, or may conform to the same standard butuse different parameters for encoding. In the similar manner, therecording unit 2908 may perform the recording in the following steps:(i) converting the original audio data stream included in themultiplexed data stream obtained by the demodulation unit 2902 into anew audio data stream, which has been encoded with an audio encodingmethod that is different from an audio encoding method performed on theoriginal audio data stream, so that the data size or bit rate of the newaudio data stream is smaller/lower than the data size or bit rate of theoriginal audio data stream; and (ii) recording a new multiplexed datastream obtained by multiplexing the post-conversion new audio data.

As one example, the stream input/output unit 2903 and the signalprocessing unit 2904 perform the processing for converting the originalvideo data stream and audio data stream included in the multiplexed datastream obtained by the demodulation unit 2902 into a new video datastream and a new audio data stream that have different data sizes or bitrates from the original video data stream and audio data stream. Morespecifically, with an instruction from the controller such as CPU, thestream input/output unit 2903 demultiplexes the multiplexed data streamobtained by the demodulation unit 2902 into demultiplexed data streams,such as a video data stream, an audio data stream, and other contentsprovided from data broadcast services. With an instruction from thecontroller, the signal processing unit 2904 performs (i) processing forconverting the demultiplexed original video data stream into a new videodata stream that has been encoded with a video encoding method that isdifferent from a video encoding method performed on the original videodata stream, and (ii) processing for converting the separated originalaudio data stream into a new audio data stream that has been encodedwith an audio encoding method that is different from an audio encodingmethod performed on the original audio data stream. With an instructionfrom the controller, the stream input/output unit 2903 generates a newmultiplexed data stream by multiplexing the post-conversion new videodata stream and the post-conversion new audio data stream. With theinstruction from the controller, the signal processing unit 2904 mayperform the conversion processing on one or both of the original videodata stream and the original audio data stream. Furthermore, the datasizes or bit rates of the post-conversion new video data stream and thepost-conversion new audio data stream may be determined by the user, ormay be predetermined for each type of recording media.

With the above structure, the receiver 2900 can perform the recordingafter changing the data sizes or bit rates of the video data stream andthe audio data stream in accordance with the data size of data that canbe recorded on the recording medium, or in accordance with the speed atwhich the recording unit 2908 records/reads data. This way, therecording unit can record a program even when the data size of data thatcan be recorded on the recording medium is smaller than the data size ofthe multiplexed data stream obtained by the demodulation unit 2902, orwhen the speed at which the recording unit records/reads data is slowerthan the bit rate of the multiplexed data stream obtained by thedemodulation unit 2902. Consequently, the receiver 2900 can read therecorded data and the user can view the program corresponding to therecorded data at any time after the broadcast time of the program.

The receiver 2900 further includes a stream output IF (interface) thattransmits the multiplexed data stream obtained by the demodulation unit2902 to an external device via a communication medium 2930. One exampleof the stream output IF 2909 is a wireless communication device thattransmits the demodulated multiplexed data to the external device via awireless medium (equivalent to the communication medium 2930), by usinga wireless communication method conforming to the wireless communicationstandards such as Wi-Fi (registered trademark) (e.g., IEEE 802.11a, IEEE802.11b, IEEE 802.11g, and IEEE 802.11n), WiGig, WirelessHD, Bluetooth,and ZigBee. Alternatively, the stream output IF 2909 may be a wiredcommunication device that transmits the demodulated multiplexed datastream to the external device via a wired communication channel(equivalent to the communication medium 2930) connected to the streamoutput IF 2909, by using a communication method conforming to the wiredcommunication standards such as Ethernet, USB (Universal Serial Bus),PLC (Power Line Communication), and HDMI (High-Definition MultimediaInterface).

With the above structure, the user can use, on the external device, themultiplexed data stream that has been received by the receiver 2900using the reception methods described in the above embodiments. Notethat the use of the multiplexed data by the user includes (i) real-timeviewing of the multiplexed data stream by using the external device,(ii) recording of the multiplexed data stream with a recording unitprovided in the external device, and (iii) transmission of themultiplexed data stream from the external device to yet another externaldevice.

It has been described above that in the receiver 2900, the output IF2909 outputs the multiplexed data stream obtained by the demodulationunit 2902. Alternatively, part of the multiplexed data may be extractedand output. For instance, when the multiplexed data stream obtained bythe demodulation unit 2902 includes contents etc. that are provided fromdata broadcast services and that are different from the video datastream and the audio data stream, the stream output IF 2909 may output anew multiplexed data stream that is obtained by extracting andmultiplexing the video data stream and audio data stream in themultiplexed data stream obtained by the demodulation unit 2902.Alternatively, the stream output IF 2909 may output a new multiplexeddata stream that is obtained by multiplexing one of the video datastream and audio data stream included in the multiplexed data streamobtained by the demodulation unit 2902.

As one example, the stream input/output unit 2903 performs theprocessing for extracting and multiplexing part of the multiplexed datastream obtained by the demodulation unit 2902. More specifically, withan instruction from the controller such as CPU (Central Processing Unit,not illustrated), the stream input/output unit 2903 generates a newmultiplexed data stream by (i) demultiplexing the multiplexed datastream obtained by the demodulation unit 2902 into demultiplexed datastreams, such as a video data stream, an audio data stream, and othercontents provided from data broadcast services, and (ii) extracting andmultiplexing only a specified data stream out of the demultiplexed datastreams. Which data stream should be extracted from the demultiplexeddata streams may be determined by the user, or may be predetermined foreach type of the stream output IF 2909.

With the above structure, the receiver 2900 can extract and output onlythe data required by the external device. This can reduce thecommunication band consumed by outputting the multiplexed data.

It has been described above that the stream output IF 2909 outputs themultiplexed data stream obtained by the demodulation unit 2902.Alternatively, the stream output IF 2909 may perform the output in thefollowing steps: (i) converting the original video data stream includedin the multiplexed data stream obtained by the demodulation unit 2902into a new video data stream, which has been encoded with a videoencoding method that is different from a video encoding method performedon the original video data stream, so that the data size or bit rate ofthe new video data stream is smaller/lower than the data size or bitrate of the original video data stream; and (ii) outputting a newmultiplexed data stream obtained by multiplexing the post-conversion newvideo data stream. The video encoding methods that are respectivelyperformed on the original video data stream and the post-conversion newvideo data stream may conform to different standards, or may conform tothe same standard but use different parameters for encoding. In thesimilar manner, the stream output IF 2909 may perform the output in thefollowing steps: (i) converting the original audio data stream includedin the multiplexed data stream obtained by the demodulation unit 2902into a new audio data stream, which has been encoded with an audioencoding method that is different from an audio encoding methodperformed on the original audio data stream, so that the data size orbit rate of the new audio data stream is smaller/lower than the datasize or bit rate of the original audio data stream; and (ii) outputtinga new multiplexed data stream obtained by multiplexing thepost-conversion new audio data stream.

As one example, the stream input/output unit 2903 and the signalprocessing unit 2904 perform the processing for converting the originalvideo data stream and audio data stream included in the multiplexed datastream obtained by the demodulation unit 2902 into new video data andaudio data that have different data sizes or bit rates from the originalvideo data stream and audio data stream. More specifically, with aninstruction from the controller, the stream input/output unit 2903demultiplexes the multiplexed data stream obtained by the demodulationunit 2902 into demultiplexed data streams, such as a video data stream,an audio data stream, and other contents provided from data broadcastservices. With an instruction from the controller, the signal processingunit 2904 performs (i) processing for converting the demultiplexedoriginal video data stream into a new video data stream that has beenencoded with a video encoding method that is different from a videoencoding method performed on the original video data stream, and (ii)processing for converting the demultiplexed original audio data streaminto a new audio data stream that has been encoded with an audioencoding method that is different from an audio encoding methodperformed on the original audio data stream. With an instruction fromthe controller, the stream input/output unit 2903 generates a newmultiplexed data stream by multiplexing the post-conversion new videodata stream and the post-conversion new audio data stream. With theinstruction from the controller, the signal processing unit 2904 mayperform the conversion processing on one or both of the original videodata stream and the original audio data stream. Furthermore, the datasizes or bit rates of the post-conversion new video data stream and thepost-conversion new audio data stream may be determined by the user, ormay be predetermined for each type of the stream output IF 2909.

With the above structure, the receiver 2900 can perform the output afterchanging the bit rates of video data and audio data in accordance withthe speed at which communication is performed with the external device.This way, the stream output IF can output a new multiplexed data streamto the external device even when the speed at which the communication isperformed with the external device is slower than the bit rate of themultiplexed data stream obtained by the demodulation unit 2902.Consequently, the user can use the new multiplexed data stream onanother communication device.

The receiver 2900 further includes the AV output IF 2911 that outputs,to the external device and an external communication medium, a videosignal and an audio signal decoded by the signal processing unit 2904.One example of the AV output IF 2911 is a wireless communication devicethat transmits the modulated video signal and audio signal to theexternal device via a wireless medium, by using a wireless communicationmethod conforming to the wireless communication standards such as Wi-Fi(registered trademark) (e.g., IEEE 802.11a, IEEE 802.11b, IEEE 802.11g,and IEEE 802.11n), WiGig WirelessHD, Bluetooth, and ZigBee.Alternatively, the AV output IF 2911 may be a wired communication devicethat transmits the modulated video signal and audio signal to theexternal device via a wired communication channel connected to the AVoutput IF 2911, by using a communication method conforming to the wiredcommunication standards such as Ethernet, USB, PLC and HDMI.Alternatively, the AV output IF 2911 may be a terminal for connecting toa cable that outputs the video signal and audio signal as-is, i.e., asanalog signals.

With the above structure, the user can use the video signal and audiosignal decoded by the signal processing unit 2904 on the externaldevice.

The receiver 2900 further includes an operation input unit 2910 thatreceives input of a user operation. The receiver 2900 switches betweenvarious operations based on a control signal that is input to theoperation input unit 2910 in accordance with the user operation. Forexample, the receiver 2900 switches between (i) ON and OFF of the power,(ii) channels to be received, (iii) display and non-display ofsubtitles, (iv) languages to be displayed, (v) volumes of audio to beoutput from the audio output unit 2906. The receiver 2900 also changesvarious settings, such as channels that can be received.

The receiver 2900 may have the function of displaying an antenna levelindicating the reception quality of signals that are being received byitself. The antenna level is an index showing the reception qualitycalculated based on, for example, RSSI (Received Signal StrengthIndication/Indicator, which indicates the strength of the receivedsignals), a reception electric field strength, C/N (carrier-to-noisepower ratio), BER (Bit Error Rate), a packet error rate, a frame errorrate, and channel state information of the signals received by thereceiver 2900. The antenna level is a signal indicating the signal leveland the quality (superior, inferior, etc.) of the received signals. Inthis case, the demodulation unit 2902 has functions of a receptionquality measurement unit that measures RSSI, the reception electricfield strength, C/N, BER, the packet error rate, the frame error rate,the channel state information, etc. of the received signals. Thereceiver 2900 displays the antenna level (the signal indicating thesignal level and the quality (superior, inferior, etc.) of the receivedsignals) on the video display unit 2907 in format that can bedistinguished by the user. The display format of the antenna level (thesignal indicating the signal level and the quality (superior, inferior,etc.) of the received signals) may display numerical valuescorresponding to RSSI, the reception electric field strength, C/N, BER,the packet error rate, the frame error rate, the channel stateinformation, etc., or may display different images in accordance withRSSI, the reception electric field strength, C/N, BER, the packet errorrate, the frame error rate, the channel state information, etc.

The following explains an exemplary method of calculating an antennalevel from the signals received by the receiver 2900 that uses thetransmission methods described in the above embodiments. By using themethods described in the above embodiments, a 2D interpolation unit 1550(not illustrated in FIG. 29) of the receiver 2900 interpolates signalsof pilots that have been detected according to the SP pattern for eachsubset. The reception quality measurement unit of the receiver 2900calculates an interpolation error, which is an error between theinterpolated values and the signals of the CPs that have been actuallyreceived, by using (i) the signals of the received CPs at CP-bearingcells, and (ii) the interpolated values of subsets to which these CPsbelong. It is considered that the smaller the calculated interpolationerror, the higher the reception quality. Thus, the receiver 2900generates an index indicating the reception quality based on thecalculated interpolation error, and displays the index as an antennalevel. At this time, as the index indicating the reception quality, thereceiver 2900 may use an average value or the largest value ofinterpolation errors calculated from the CPs within a predetermined unitof time. In addition, the interpolation error may be expressed using anabsolute value or a value normalized by the reception power.

When the Hadamard transform is used to encode pilot signals, thereception quality measurement unit of the receiver 2900 may beconfigured as follows. By using (i) values of signals separated with useof channel components h₁, . . . , h_(n), which are calculated byperforming the Hadamard transform (where n is an integer greater than orequal to two and is the same as the number of transmit antennas), and(ii) values of known CPs transmitted from antennas of each transmitter,the reception quality measurement unit of the receiver 2900 calculatesan error included in the separated signals. The receiver 2900 generatesan index indicating the reception quality based on the calculated errorincluded in the separated signals, and displays the index as an antennalevel. At this time, as the index indicating the reception quality, thereceiver 2900 may use an average value or the largest value of errorscalculated within a predetermined unit of time as included in theseparated signals.

With the above structure, in a case where signals are received by usingthe reception methods described in the above embodiments, the user cangrasp the antenna level (the signal indicating the signal level and thequality (superior, inferior, etc.) of the received signals) eithernumerically or visually.

Furthermore, regarding methods of displaying the antenna level, thereceiver 2900 may be configured as follows. Although it is not necessaryto combine the following several methods of displaying the antenna levelwith the SP pattern described in the above embodiments, it goes withoutsaying that such a combination is expected to improve the receptionquality.

For example, the receiver 2900 may have functions of (i) calculatingindices indicating reception qualities of the separated signals,respectively, and (ii) displaying the indices as multiple antenna levels(signals indicating the signal levels and the qualities (superior,inferior, etc.) of the respective separated signals), either all at onceor by switching from display of one index to display of another index.Alternatively, the receiver 2900 may have functions of (i) calculatingan index indicating the reception quality of a group including all orsome of the separated signals, and (ii) displaying the index as theantenna level (the signal indicating the signal level and the quality(superior, inferior, etc.) of the respective separated signals).

Furthermore, the receiver 2900 may be configured as follows in a casewhere the broadcast station (base station) 2801 incorporates multipletransmission modes such as MISO and SISO (Signal Input Single Output)other than MIMO explained in the above embodiments and performs thetransmission while switching from one transmission mode to another overtime. For example, the receiver 2900 may have functions of (i)calculating indices indicating reception qualities of the multipletransmission modes, respectively, and (ii) displaying the indices asmultiple antenna levels (signals indicating the signal levels and thequalities (superior, inferior, etc.) of the respective receivedsignals), either all at once or by switching from display of one indexto display of another index. Alternatively, the receiver 2900 may havefunctions of (i) calculating an index indicating the reception qualityof a group including all or some of the multiple transmission modes, and(ii) displaying the index as the antenna level (the signal indicatingthe signal level and the quality (superior, inferior, etc.) of therespective received signals).

Furthermore, the receiver 2900 may be configured as follows in a casewhere the broadcast station (base station) 2801 groups a plurality ofdata streams that constitute a program (e.g., a video data stream and anaudio data stream) into a plurality of hierarchical layers and performsthe transmission by using a hierarchical transmission method in which atransmission mode, a modulation method, error correction encoding, anencoding rate, etc. are independently configurable for each hierarchicallayer. For example, the receiver 2900 may have functions of (i)calculating indices indicating the reception qualities of hierarchicallayers, respectively, and (ii) displaying the indices as multipleantenna levels (signals indicating the signal levels and the qualities(superior, inferior, etc.) of the received signals), either all at onceor by switching from display of one index to display of another index.Alternatively, the receiver 2900 may have functions of (i) calculatingan index indicating the reception quality of a group including all orsome of the multiple hierarchical layers, and (ii) displaying the indexas the antenna level (the signal indicating the signal level and thequality (superior, inferior, etc.) of the received signals).

With the above structure, in a case where signals are received by usingthe reception methods described in the above embodiments, the user cangrasp the antenna level (the signal indicating the signal level and thequality (superior, inferior, etc.) of the received signal), eithernumerically or visually, in units of reception that can bedistinguishable (e.g., the separated signals, multiple transmissionmode, and multiple hierarchical layers).

In an exemplary case described above, the receiver 2900 includes theaudio output unit 2906, the video display unit 2907, the recording unit2908, the stream output IF 2909, and the AV output IF 2911. However, thereceiver 2900 need not include all of these structural elements. As longas the receiver 2900 includes at least one of these structural elements,the user can use the multiplexed data stream obtained throughdemodulation by the demodulation unit 2902 and error correctiondecoding. Therefore, each receiver may include any combination of theabove structural elements depending on how it is used.

The present invention is useful in a communication system where multipletransmit antennas transmit signals at the same time in the samefrequency band and the transmitted signals are received and demodulated.

1-17. (canceled)
 18. A multi-antenna OFDM transmitter having N antennas,N being an integer greater than or equal to two, the multi-antenna OFDMtransmitter comprising: a multi-antenna encoder which, in operation,generates a plurality of data streams, one for each of the N transmitantennas, each data stream consisting of a succession of OFDM symbols,each OFDM symbol consisting of a plurality of OFDM cells, each OFDM cellbeing associated with one of a plurality of subcarriers; anon-transitory memory; and a processor coupled to the non-transitorymemory, wherein the processor, in operation, generates, for each of theplurality of data streams, a plurality of scattered pilots and continualpilots, said plurality of scattered pilots and continual pilots beingpartitioned into M subsets, each of the scattered pilots being encodedon the basis of the subset to which the scattered pilot belongs and thedata stream into which the scattered pilot is to be inserted, and theprocessor, in operation, inserts, for each of the plurality of datastreams, the plurality of scattered pilots into the data stream inaccordance with a predetermined pilot allocation pattern in which atemporal spacing between two OFDM symbols having scattered pilots inOFDM cells associated with the same subcarrier is equal to Ds, each ofDs being an integer greater than or equal to two, M is greater than orequal to N and satisfies a relationship M=M_(s)M_(k), each of M_(s) andM_(k) being an integer greater than or equal to two, and the insertionof the plurality of scattered pilots into the data stream is performedin such a manner that the scattered pilots of the same subset areallocated every M_(s) pilot cells associated with the same subcarrierand the scattered pilots of the same subset are borne every M_(k) pilotbearing subcarriers and the continual pilots are evenly distributedamong the M subsets, the pilot cells being the OFDM cells to which thescattered pilots are allocated, and the pilot bearing subcarriers beingthe subcarriers bearing the scattered pilots.
 19. An OFDM receivercomprising: receiver to which, in operation, a received signalconsisting of a succession of OFDM symbols is input, each OFDM symbolconsisting of a plurality of OFDM cells, each OFDM cell being associatedwith one of a plurality of subcarriers; a non-transitory memory; and aprocessor coupled to the non-transitory memory, wherein the processor,in operation, (i) extracts scattered pilots and at least one continualpilot from the received signal in accordance with a predetermined pilotallocation pattern, and (ii) partitions the extracted scattered pilotsand the extracted continual pilot into M subsets, the processor, inoperation, estimates a plurality of channel components from the Msubsets of scattered pilots and continual pilots, each channel componentrepresenting a channel condition between one of a plurality oftransmitters and the OFDM receiver, M satisfies a relationshipM=M_(s)M_(k), each of M_(s) and M_(k) being an integer greater than orequal to two, and the scattered pilots of the same subset are allocatedevery M_(s) pilot cells associated with the same subcarrier and thescattered pilots of the same subset are borne every M_(k) pilot bearingsubcarriers, the continual pilots being evenly distributed among the Msubsets, the pilot cells being the OFDM cells to which the scatteredpilots are allocated, and the pilot bearing subcarriers being thesubcarriers bearing the scattered pilots.